A resinous polymer of butadiene homopolymer or butadiene-styrene copolymer has been introduced to the market as Dienite. It has a butadiene content of 60 per cent or more by weight, and at least 80% by weight of the butadiene therein is of the 1,2-configuration. Dienite is added to polyesters of unsaturated aliphatic acids to increase their heat distortion temperatures and resistance to hydrocarbon solvents and aqueous sodium hydroxide. The flow and handling of such blends is improved by including styrene.